Two selenium-containing enzymes from Clostridium kluyveri are being characterized. Thiolase and Beta-hydroxybutyryl-CoA dehydrogenase are major enzymes from this anaerobic bacteria, and both have been found to contain selenium. These proteins constitute a new clas of selenium-containing enzymes, in that they contain the element as selenomethionine. Most of the other seleno-proteins have been shown to contain selenocysteine. A rapid one-step isolation precedure has been developed for the isolation of the enzymes from crude extracts. Antibodies have been raised to both enzymes and can be used to monitor the incorporation of selenium into these proteins under a variety of growth conditions. Thiolase has been shown to incorporate selenium even in the presence of high concentrations of methionine. The distribution of selenium in thiolase has also been investigated and a specific selenium-containing subunit has been isolated by reversed-phase high pressure liquid chromatography.